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13 Nuclear Magnetic Resonance Spectroscopy -- Part 6
The acid-catalyzed opening of other epoxides is similar to the cyclization of squalene-2,3-epoxide.
The oxygen is attacked by a nucleophile.
The pi bond is the nucleophile.
The initial result is a tertiary carbocation.
cyclization of the carbon skeleton is promoted by the opening of this epoxide.
Lanosterol is converted to other steroids after the cyclized intermediate is converted.
The acid-catalyzed opening of the epoxide is the beginning of cyclization.
Another carbocation is formed by each additional cyclization step.
A compound with four rings and seven used in antifungal drugs is made from achiral, a starting material of squalene epoxidase.
The sequence has high yields lete's foot, jock itch, ringworm, and complete stereospecificity.
An alkoxide ion is a poor leaving group, so most ethers don't undergo nucleophilic substitution or eliminations.
The ring strain that is released upon opening is enough to compensate for the poor alkoxide leaving group.
The ether has a lower activation energy than the starting epoxide, which has a higher energy than ether.
The same product as the acid-catalyzed opening of the epoxide is produced by the same reaction.
Under milder conditions, the acid-catalyzed reaction may be used to open an epoxide.
The acid-catalyzed hydrolysis is preferred if there is an acid-sensitive functional group present.
Alkoxide ion react with epoxides to form ring-opened products.
The same trans-2-methoxycyclopentanol is produced in the acid-catalyzed opening in methanol when cyclopentene oxide reacts with sodium methoxide.
Most ethers are not attacked or cleaved by strong bases and nucleophiles.
The strain of the three-membered ring is relieved by opening the epoxide.
Even though the leaving group is an alkoxide, strong bases can attack.
The ring is opened by a strong base attack.
The diol comes from the alkoxide.
Amine can open epoxides.
Ethanolamine is an important industrial reagent when ene oxide reacts with ammonia.
Diethanolamine and triethanolamine can be given by the nitrogen atom in ethanolamine.
Excess ammonia can be used to achieve good yields of ethanolamine.
There is a complete mechanism for the reaction of cyclopentene oxide with sodium methoxide.
Predict the major product when each reagent reacts.
The same product can be found in both the acid-catalyzed and base-catalyzed ring openings.
Different products may be produced under acid-catalyzed and base-catalyzed conditions.
The less carbon atom is attacked by the alkoxide ion under basic conditions.
The alcohol attacks the protonsated epoxide.
It might seem that alcohol would attack at the less hindered oxirane carbon, but this is not the case.
There is a balancing act between ring strain and the energy it costs to put some of the positive charge on the carbon atoms.
If the ring started to open, we could use resonance forms to show what the cations would look like.
The "no-bond" resonance forms help us to see the charge distribution.
The structures I and III show that the oxirane carbon atoms share part of the positive charge.
structure I is more important than structure III because the tertiary carbon bears a larger part of the positive charge.
The lower transition state energy for attack at the tertiary carbon is implied by the weaker bond between the tertiary carbon and oxygen.
Attack by the weak nucleophile is sensitive to the strength of the electrophile, and it occurs at the tertiary carbon.
The opening of a bromonium ion in the formation of a bromohydrin and the opening of the mercurinium ion during oxymercuration are similar to this ring opening.
All three reactions involve the opening of a ring.
Attack takes place at the more carbon atom, which is the more carbon that supports the positive charge.
Most base-catalyzed epoxide openings involve attack by a strong nucleophile at the less hindered carbon atom.
The less substituted carbon is less hindered by base-catalyzed attacks.
The alcohol attacks the tertiary carbon atom of the protons.
Predict the major products of the reactions.
ethylmagnesium bromide reacts with oxirane to form magnesium salt of butan-1-ol.
The neutral alcohol is given by particleation.
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